I have often seen this poem referred to as "What happens to a dream deferred" rather than "Harlem," which I think is very interesting when you think about the poem's theme.
The major theme for me is one of civil rights and the anger and frustration inherent in the struggle for them (see the second link below). This is implied through the title. Harlem, of course, was a hotbed for civil rights during the 50's and 60's (see the third link below). Of course, the first line is an allusion to Dr. King's famous "I Have a Dream" speech about racial equality. The poem then posits what will happen if we continue to defer or postpone King's dream of racial equality.
The final line, "Or does it explode" suggests to the reader that if the dream is always postponed, then violence will erupt not only in Harlem but also across the country wherever civil rights are repressed.
Langston Hughes is the figure most frequently associated with the Harlem Renaissance or New Negro Movement. Harlem was the black mecca for quite sometime and then became over shadowed by the vices that tempt all reach new heights of wealth and fame. The dream of representing the black community's talent, intelligence and dedication to communal growth was overshadowed by the drugs, gang activity and untimely deaths of key figures. The dream was set aside. Harlem was the dream and many of the metaphoric lines in the poem could be used to describe those who lived, worked and dreamed there.
Look up the Talented Tenth Philosophy by WEB DuBois... I think Langston saw it coming.
I'll post a less cynical view next :)
i know you received an answer three months ago so I hope you are still interested in a little clarification about the answer you received. As kreynolds6931 responded, the theme of the poem "Harlem" may be about civil rights and anger and frustration but the first line can not be an allusion to Dr. King's speech as the poem was written in the 1950's and Dr.King's speech wasn't given until 1968.